Positioner for a valve that can be actuated by a drive

ABSTRACT

A positioner for a valve that is actuated by means of a drive. The positioner includes a locator ( 9 ) and a control unit ( 13 ). The locator ( 9 ) detects the actual position of an actuator ( 7 ). The control unit ( 13 ) compares the actual position with a predefined desired position, and generates an actuating signal. A magnet ( 18 ) having a magnetoresistive sensor, preferably a GMR sensor, is provided as the locator. The locator ( 9 ) is less susceptible to dirt and less prone to wear and tear than a conventional slide potentiometer. The positioner is thus less fault-prone.

This is a Continuation of International Application PCT/DE01/01283, withan international filing date of Apr. 2, 2001, which was published underPCT Article 21(2) in German, and the disclosure of which is incorporatedinto this application by reference.

FIELD OF AND BACKGROUND OF THE INVENTION

The invention relates to a positioner, especially for a valve that canbe actuated by a drive.

European Publication EP 0 637 713 A1 discloses such a positioner for avalve that is actuated by a drive. The valve is installed in a pipe andcontrols the passage of a medium by way of a corresponding stroke of aclosing element that interacts with a valve seat. A pneumatic drive isconnected, by a push rod, with the closing element. A lever engages withthe push rod and acts on a potentiometer, which functions as a locatorof the positioner. The potentiometer detects the actual position of theactuator. A control unit of the positioner compares this actual positionwith a predefined desired position. As a function of the determineddeviation, the control unit generates an actuating signal to control thepneumatic drive. The desired value is predefined for the positionerthrough a normalized signal, e.g., a 4 to 20 mA interface or a digitalfield bus message. Thus, the role of the positioner is to convert thepredefined desired value of the actuator position into a pneumaticpressure signal that is supplied to the pneumatic drive and results in acorresponding position of the push rod.

In addition, flap valves are known in the art in which the opening angleof a rotary valve is detected by means of a rotary potentiometer. Inthis case, a positioner generates an actuating signal for a rotaryactuator that controls the rotary valve.

Slide potentiometers, because of their simple and inexpensiveconstruction, are frequently used for position detection. Theiradvantage is that they produce a usable electrical actuating signal in arelatively simple manner with low power consumption. For instance, a 10kΩ potentiometer operated at 3 V consumes a maximum of 300 μA. Thestroke or rotary movement of the actuator is applied to thepotentiometer's axis of movement via corresponding add-on parts, e.g., arotary lever with a switchable gear drive, and the component voltagedetected by the potentiometer is transmitted to the analog input of ananalog or digital control unit. The detection range of the angle ofrotation for rotary actuators is typically 120° maximum. For linearactuators, typically the detection range is 15 mm maximum. The linearmotion can also be converted into an angle of rotation of 120° maximumby means of a conversion mechanism.

In many areas of process and power technology, the fault-free operationof a plant depends on the flawless functioning of the control valvesused. Downtimes of plants or plant parts caused by component failuressignificantly reduce the production capacity and the possibleutilization of the plant. Thus, reducing downtimes and increasing systemreliability are essential goals for efficient plant operation.

Due to their construction, the electromechanical slide potentiometers,which are frequently used for rotary or linear position detection, havedrawbacks regarding their long-term stability because of wear andoxidation of the contact paths as well as because of their vibrationfatigue limit. After prolonged quasi-static operation, their sliderstend to stick. Due to mechanical wear, the sliders and the resistivecoatings eventually wear or their quality changes as a result of agingand oxidation. In electromechanical slide potentiometers, the rotary orlinear motion is transmitted by means of a continuous shaft. Suitableencapsulation against environmental influences is therefore very costlyand in itself is susceptible to aging and wear.

European Patent EP 0 680 614 B1 discloses a device for detecting anangular position of an object. The sensors described in this patentspecification are based on the giant magnetoresistive (GMR) effect andconsist of alternating magnetically hard and magnetically soft metallayers. These layers are each only a few atoms thick and are sputteredonto a silicon substrate. The resistance of the sensors greatly dependson the direction of a magnetic field acting on them. A GMR sensor isthus very well suited to detect a change in the angular position of amagnet.

OBJECTS OF THE INVENTION

An object of the invention is to provide a positioner, particularly fora valve that is actuated by a drive, which is distinguished by itsimproved interference immunity while being inexpensive to produce.

SUMMARY OF THE INVENTION

To attain this and other objects, according to the principles of thepresent invention and according to one formulation, the novelpositioner, for a valve (2) that is actuated by a drive (6), includes: alocator (9) that detects the actual position of an actuator (7), and acontrol unit (13) that compares the actual position with a predefineddesired position and generates an actuating signal. The locator includesa permanent magnet (18) and a sensor (50), and the magnet and sensor arerotatable or displaceable relative to one another in conjunction with amovement of the actuator (7). Further, the sensor (50) is arranged in anarea of a housing (90) such that the sensor (50) is positioned to detecta relative rotation between the sensor and the magnet when the magnetrotates about an axis of rotation, and is positioned to detect arelative shift between the sensor and the magnet when the magnet isdisplaced, wherein the shift occurs in a plane that extendssubstantially perpendicularly to the axis of rotation.

The invention obviates the drawbacks of conventional potentiometers,since it uses a contactless potentiometer that includes a magnet and amagnetoresistive sensor. The novel locator provides the exact actualposition of the actuator in either a dynamic or a static case. Anon-linearity of the locator's output signal, which is minor in anycase, is readily compensated. Between the magnet and themagnetoresistive sensor, a partition can easily be installed forencapsulation and, thus, protection against environmental influences.Therefore, the locator is very rugged and insensitive to dirt and aharsh environment. The magnet is easily mounted outside the sensorhousing on a linear or rotary actuator such that its magnetic fieldlines act on the magnetoresistive sensor through the housing wall. Anevaluation circuit is readily integrated in the sensor housing. Thisevaluation circuit generates a voltage proportional to the angle ofrotation, or the linear path, of the magnet by way of the change inresistance of the magnetoresistive sensor. Thus, the evaluation circuitsupplies, to a control unit, a signal that corresponds to the actualposition and is immune to interference.

A minimum distance between the magnet and the sensor is easily kept toprevent damage to the magnetically hard layers, especially in a GMRsensor, since in this sensor type the strength of the magnetic field maynot exceed 15 kA/m. The contactless principle of the novel locatoreliminates the problem of a scratching or sticking slide potentiometer.This contactless principle offers advantages in applications where thepotentiometer is exposed to continuous vibrations. It is alsoadvantageous in the quasi-static case where the potentiometer positionremains unchanged over a long period of time, and where there is a riskthat the slider of a slide potentiometer would dig into the resistancelayer and possibly get stuck there due to control instability in thesystem. If the magnet forms the moving part of the locator, which iscoupled with the actuator, it couples the actuating movement into themagnetoresistive sensor through its magnetic field, without requiringany mechanical duct. By corresponding add-on parts, an exact rotary orlinear motion of the moving part is ensured in a simple manner.

If the magnet is designed as a permanent magnet, a particularly simplestructure results, since the magnet does not require a power supply, andthus does not increase the current consumption of the locator.

An advantageous clear increase in the resistance of the magnetoresistivesensor results if a so-called anisotropic magnetoresistive sensor isused. When the magnetization of the layer is rotated relative to thecurrent direction of a measuring current flowing through the layer ofthe sensor, there is a change in the resistance in this type of sensor,which can be a few percentage points of the normal isotropic resistance.This ensures a sufficiently high signal-to-noise ratio of themeasurement signal.

Using a so-called giant magnetoresistive (GMR) sensor has the advantagethat the change in the resistance is independent of the field strengthwithin a wide range, and is only sensitive to the direction of themagnetic field. This directional dependence of the resistance resemblesa cosine function, and is therefore nearly linear within a wide range.

Advantageously, the same sensor construction can be used forinstallation, in both rotary actuators and linear actuators, withoutrequiring any structural changes. For this purpose, the GMR sensor isarranged in the area of the edge of a housing in such a way that thesame sensor is positioned to detect a relative rotational movement atleast approximately on the axis of rotation of a magnet that is providedfor this case, and to detect a relative shift jointly with a magnet thatis provided for this case in a plane that extends substantiallyperpendicularly to the aforementioned axis of rotation. The distancebetween the sensor and the housing wall facing the magnet is preferablyabout 5 mm. This ensures that the required minimum distance betweenmagnet and sensor is met. Since the sensor can be used in both rotaryand linear actuators, the costs of logistics and warehousing are reducedbecause only one GMR sensor type is required.

Improved measurement accuracy in case of temperature fluctuations isobtained by arranging a temperature compensation circuit in the housingof the GMR sensor. To obtain particularly good temperature compensation,the bridge resistance of the GMR sensor is simultaneously used as ameasurable resistance for the temperature compensation circuit. Thiscompletely eliminates problems of thermal coupling between the measuringresistor and the GMR sensor.

Advantageously, the GMR sensor is arranged on one side and thetemperature compensation circuit is on the other side of the sameprinted circuit board. As a result, the components of the temperaturecompensation circuit, the housing of which is typically larger than thecomponent housing of the GMR sensor, do not need to be arranged betweenthe GMR sensor and the exterior of the locator housing that faces themagnet. Therefore, the components of the temperature compensationcircuit do not influence the spacing between the GMR sensor and theexterior of the locator housing. This makes it possible to keep a smalldistance between the upper edge of the component housing of the GMRsensor and the locator-housing exterior.

Precise positioning of the magnet relative to the GMR sensor can easilybe achieved by providing a centering aid on the housing of the GMRsensor to adjust the relative position of the magnet in relation to thesensor during installation. This positioning aid is configured as amolded part that is placed on the magnet, removed again afterinstallation, and that is inserted into an opening on the housing of theGMR sensor in a positive fit during installation. After the magnet andthe GMR sensor have been attached, the molded part is removed.

A mechanically positive-locking configuration of the moving part and thesensor housing ensures the spatially correct positioning of the magnetand the sensor. The connections of the two parts to form a completelocator can be non-positive, i.e., wireless. Alternatively, the locatorcan be constructed as a complete, mechanically integral, locator block,which comprises the moving part with magnet, GMR sensor and evaluationelectronics. Such a locator ensures a defined distance between themagnet and the GMR sensor. In principle, active evaluation electronicsthat are completely isolated from the moving part, both mechanically andelectrically, make possible robust and interference-free locatorelectronics in miniature form that are screened in a simple manneragainst electrical as well as magnetic interference. The magnet itselfrequires no mechanical duct through a partition to the housing of theGMR sensor and is located together with the GMR sensor in a commonscreened chamber for protection against electrostatic andelectromagnetic interference. For applications in areas that are subjectto extreme interference, a corresponding external screen, which alsoencloses the magnet, can be constructed as an add-on component ifrequired.

To prevent the control unit from being exposed to possibly hightemperatures that may prevail in the locator, the control unit isadvantageously arranged in a second housing separate from the GMR sensorhousing. In this case, the locator and the control unit areinterconnected by a medium for transmitting the actual position of theactuator, e.g., by an electric cable. The linear or rotary movement isdetected directly by the locator in a sensor housing, which is attachedon the drive or the actuator by a corresponding adapter kit. The sensorhousing can also be formed by the housing of a positioner, in which onlythe circuit parts of the locator are arranged for separate construction.The control unit of the positioner is installed at some distance, e.g.,on an installation pipe or a similar installation aid, and is connectedwith the locator by an electric cable and with the pneumatic drive byone or two pneumatic lines. It is useful to accommodate the locator andthe control unit in separate housings, particularly if the environmentalconditions around the actuator exceed the values specified for thecontrol unit. This may be the case, for instance, if the temperatures onthe valve or the drive are high because of a hot medium flowing thevalve, or if the valve or the drive is subject to strong oscillations orvibrations, or if there is little room available on the valve or thedrive to mount the entire positioner.

Other advantages of the novel positioner include its: suitability inareas subject to explosion hazards due to its low power requirements andreadily integratable protective circuits; wide supply voltagetolerances; minimized external interference through integrated screensand EMI filters; minimized temperature influences with low supplycurrent; minimized and stably reproducible hysteresis as a function ofthe angle of rotation between magnet and GMR sensor; and field strength.

The minor hysteresis fluctuations and the minor non-linearity, resultingfrom manufacturing tolerances of the GMR sensors, are irrelevant for theapplication as a locator in a positioner. If the positioner is intendedto output the actual position, as information to additional componentsof a system, the output signal is easily corrected and actively filteredcorresponding to the known linearity and hysteresis characteristics ofthe individual GMR sensor. For this purpose, if required, the specificcorrection data of the GMR sensor is stored in a microcontroller of thepositioner. For simplified correction of linearity and hysteresiserrors, it is sufficient to determine and store five basicsensor-specific characteristic values, which are recorded under standardconditions. These basic values are located, for instance, at the pointsof the maximum change in the slope of the curves. For a more precisecorrection, all the characteristic curves are stored with the desiredresolution in a serially readable memory medium supplied with the GMRsensor and assigned to the GMR sensor by an identification key. Thecontent of the memory medium is loaded, for instance, into themicrocontroller upon installation of the GMR sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as embodiments and advantages thereof, will nowbe described with reference to exemplary embodiments depicted in thedrawings, in which:

FIG. 1 shows a control valve;

FIG. 2 is a block diagram of a locator;

FIG. 3 shows a circuit for temperature compensation;

FIG. 4 shows a circuit for amplification and offset adjustment;

FIG. 5 shows a printed circuit module with the circuits depicted inFIGS. 3 and 4;

FIG. 6 shows a metallic screen for the printed circuit module accordingto FIG. 5;

FIG. 7 is a top view of an open metallic screen;

FIG. 8 is a side view of an open metallic screen;

FIG. 9 is a housing for a GMR sensor;

FIG. 10 is a sealing cover for the housing depicted in FIG. 9;

FIG. 11 is a bottom view of an angular position sensor;

FIG. 12 is a side view of the angular position sensor according to FIG.11;

FIG. 13 is a bottom view of a linear position sensor; and

FIG. 14 is a side view of the linear position sensor according to FIG.13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a valve 2 that is installed in a pipe 1 of a processtechnology plant (not depicted) and controls the flow rate of a medium 5by a corresponding stroke of a closing element 4 that interacts with avalve seat 3. The stroke is produced by a pneumatic drive 6 and istransmitted by a valve rod 7 to closing element 4. Drive 6 is connectedwith the housing of valve 2 by a yoke 8. A locator 9 is mounted on yoke8 and on the input side detects the stroke of valve rod 7 by way of aconnecting element 10 guided on valve rod 7. The locator 9 generates ananalog output signal 11 that corresponds to the stroke. The pneumaticdrive 6 comprises a substantially horizontal membrane, which separatesan upper from a lower chamber. The lower chamber is connected, via apipe 12, with a control unit 13. The control unit 13 is accommodated ina housing that is separate from the housing of locator 9. A spring isarranged in the upper chamber, which acts against the pressure of thelower chamber. In the absence of pressure, the spring closes valve 2.

Through valves controlled in control unit 13, inlet air that is suppliedthrough a line 14 is introduced at a pressure P into the lower chambervia pipe 12, or is released as exhaust air into the environment via aline 15. Control unit 13 compares the actual position of valve rod 7(which is described as an actuator in control system terms), which itreceives by signal 11, with a desired value supplied by a field bus 17via a data interface 16. The control unit 13 can then correct anydeviation by correspondingly adjusting the air stream in pipe 12.

The connecting element 10 is embodied as a lever arm that is guidedbetween two pins mounted on valve rod 7. Thus, the connecting elementfollows the stroke movements of valve rod 7. A magnet 18, fixed toconnecting element 10, is rotatably supported in the housing of locator9, which also contains a GMR sensor. Through movement of connectingelement 10, the magnet 18 is set into a rotary motion corresponding tothe stroke of valve rod 7. While locator 9 is fixed to yoke 8, and mayconsequently be exposed to high ambient temperatures, control unit 13 ismounted at a distance therefrom in a less harsh environment, e.g., on aninstallation pipe (not depicted in FIG. 1). This expands the scope ofapplication of the positioner, which typically comprises sensitivevalves for pneumatic control.

FIG. 2 depicts a circuit diagram of an evaluation circuit with a GMRsensor, which is integrated in locator 9 (FIG. 1). In principle, theevaluation circuit—for measuring a change in the resistance of the GMRsensor, which depends on the direction of the magnetic field—comprises acircuit 20 for supplying the measuring bridge and compensates for achange in temperature. The evaluation circuit further comprises acircuit 21 for signal conditioning with offset formation andamplification of a bridge output signal dU, which is supplied by circuit20. Circuit 21 generates an output signal 22, e.g., with a range ofvalues from 0.1 to 2.5 V, which represents the actual position of theactuator. The output signal 22 corresponds to signal 11 in FIG. 1.Additional circuit elements, which are not depicted in FIG. 2, comprise,for example, EMI filters and redundant electronic current and voltagelimiting devices that are located in the connecting branches of thecircuit and are used for interference immunity and to avoidimpermissible operating states with respect to explosion protection. Theentire evaluation circuit is distinguished by its very low powerconsumption, which is less than 300 μA.

FIG. 3 is a more detailed diagram of circuit 20 (FIG. 2), which is usedfor temperature compensation and for operating a GMR sensor 30. The GMReffect is temperature dependant. The bridge output voltage dU can beapproximated by the following formula:${{dU}\left( {\alpha,T} \right)} = {{{{\frac{1}{2} \cdot \frac{\Delta \quad R}{R}}{\left( T_{0} \right) \cdot \left\lbrack {1 + {{Tk}_{\Delta \quad {R/{Ro\_ lin}}} \cdot \left( {T - T_{0}} \right)} + {Tk}_{\Delta \quad {R/{Ro\_ Q}}} - \left( {T - T_{0}} \right)^{2}} \right\rbrack \cdot {U_{b}(T)} \cdot {\cos (\alpha)}}} + {U_{off}{dU}(T)}} \sim {u_{B}\left\lbrack {f(T)} \right\rbrack}}$

where

α is the angle included between the direction of the magnetic field andthe GMR sensor,

T is the temperature of GMR sensor 30,

T₀ is 20° C.,

R₀ is the resistance at 20° C.

Tk_(ΔR/Ro) _(—) _(lin) and Tk_(ΔR/Ro) _(—) _(Q) are the compensationparameters and

U_(off) is an offset voltage.

To counteract a drop, due to temperature, in the bridge output voltagedU of GMR sensor 30, a supply voltage Ub of the bridge is increasedaccordingly. This function is implemented by the circuit depicted inFIG. 3. Without a resistance R_(komp), the circuit would represent aconstant current source for a current Ib whose value is adjusted by aresistor R1 and the voltage on a voltage divider, wherein the voltagedivider is formed by resistors R4 and R5 as well as R3. The voltagedivider is supplied with a voltage V_(ref)=2.5 V. The resistance of theGMR sensor bridge R_(sen) increases with the temperature. However, thevoltage dU at the bridge output, which changes with the direction of themagnetic field, drops by about twice that amount. As a consequence, thevoltage increase through the constant current source is not sufficientto keep constant the amplitude of the bridge output voltage dUindependent of the temperature. The voltage increase is thereforeadjusted by a positive feedback with resistance R_(komp) such that itcompensates the reduction of the sensor effect on the sensor bridge. Thebridge resistance of the GMR sensor 30 itself serves as a temperaturesensor. For optimal temperature compensation, R_(komp) is determined by:$R_{komp} = {\frac{{R_{sen}\left( T_{0} \right)} \cdot \left( {{R_{3}R_{4}} + {R_{3}R_{5}} + {R_{4}R_{5}}} \right)}{R_{1}\left( {R_{4} + R_{5}} \right)} \cdot \frac{1}{\left( {\frac{1}{C} - \frac{1}{D}} \right)} \cdot \quad  \cdot \left( {\frac{1}{\left\lbrack {1 - {50{Tk}_{\Delta \quad {R/{Ro\_ lin}}}} + {2500{Tk}_{\Delta \quad {R/{Ro\_ Q}}}}} \right\rbrack} - \frac{1}{\left\lbrack {1 + {60{Tk}_{\Delta \quad {R/{Ro\_ lin}}}} + {3600{Tk}_{\Delta \quad {R/{Ro\_ Q}}}}} \right\rbrack}} \right)}$whereC = [1 − 50Tk_(Δ  R/Ro_lin) + 2500Tk_(Δ  R/Ro_Q)][1 − 50Tk_(Rsen_lin) + 2500Tk_(Rsen_Q)]andD = [1 + 60Tk_(Δ  R/Ro_lin) + 3600Tk_(Δ  R/Ro_Q)][1 + 60Tk_(Rsen_lin) + 3600Tk_(Rsen_Q)]

If the values of resistors R1, R3, R4 and R5 are suitably selected, thiscircuit is distinguished by particularly low current consumption withgood accuracy of the temperature compensation.

The output signal dU of the GMR sensor 30 (FIG. 3) is adjusted by meansof the circuit shown in FIG. 4 with respect to its amplification and itsoffset adjustment. An operational amplifier 40, together with a voltagedivider is used to adjust the amplification. The operational amplifier40 is operated with a supply voltage Ucc=3 V. The voltage dividerincludes resistors R_(off) and R9, to which a reference voltage Uref=2.5V is applied. The output voltage obtained at the output of theoperational amplifier 40 is supplied to a difference amplifier 41, whichis used to adjust the amplification. This difference amplifier 41 isalso operated at a supply voltage Ucc=3 V. In this manner, thedifferential signal dU is amplified from approximately 3 mV to 1.2 V andis boosted to an average potential of 1.3 V. An output signal 42corresponding to signal 11 in FIG. 1, has a range of values from 0.1 to2.5 V. An amplifier resistance R_(gain) is selected in such a way thatthe range of values of the output signal dU of the GMR sensor 30 (FIG.3) is mapped to the range of values of the output signal 42. Thiscircuit is also distinguished by its particularly low currentconsumption. This is important especially if the locator is used incombination with a field bus, which is used to transmit both the energyrequired to operate the circuit components and the information signals.Even if a 4 to 20 mA interface is used for the positioner, low currentconsumption of the circuit components is particularly important, becausethe positioner must be able to manage with an operating current of onlyapproximately 4 mA.

FIG. 5 shows one possible spatial arrangement of a GMR sensor 50 and anevaluation circuit on a printed circuit board 52. The GMR sensor 50 isarranged on the bottom side of the printed circuit board 52 (illustratedas transparent here for better clarity) while components 51 of theevaluation circuit are mounted on the topside. As a result, the highercomponents 51 of the evaluation circuit do not need to be taken intoaccount when determining the distance between the upper edge of the GMRsensor 50 and the housing exterior. Along the front edge of printedcircuit board 52, four solder lugs 53 are provided to which cable ends54 of a cable 55 are soldered. Two wires of the cable serve to outputthe output signal (11 in FIG. 1), whereas the other two wires areconnected to the electronic circuit components of the locator.

As an alternative to the described embodiment with a GMR sensor 50, thesensor can be configured as a so-called anisotropic magnetoresistivesensor. The circuit principle of the evaluation circuit remainsunaffected.

The equipped printed circuit board 56 is inserted into a metal screen60, which is shown closed in FIG. 6 and open in FIGS. 7 and 8.

Like components in the figures are provided with like referencenumerals. For the correct positioning of printed circuit board 56, threesolder pins 61, 62 and 63 are provided, which project into correspondingholes of printed circuit board 52 where they are soldered for assembly.After the printed circuit board 52 has been soldered, the metal screenis folded and cable 55 is placed into clamping lugs 64, 65 and 66 whereit is held in place by clamping force.

In the areas where the GMR sensor 50 comes to rest, a substantiallysemicircular opening 67 is provided in the metal screen 60 so that amagnetic field can penetrate metal screen 60 and reach GMR sensor 50. Anopening 68 serves for exact positioning of metal screen 60 in a housing90, which is illustrated in FIG. 9. During insertion into housing 90,opening 68 is pushed onto a rib, which in FIG. 9 is covered up by thetopside of the housing. After centering, this rib fits into a groove 69of opening 68.

Housing 90 of GMR sensor 50 is made, for example, of a plastic materialor a non-ferromagnetic material, which protects the printed circuitboard 56 against environmental influences. At the same time, housing 90provides means for fastening the locator at the site. For simplefastening on standard mounting kits, housing 90 is provided with twolocation holes 91 and 92 for screws and with a positioning pin 93. Thepositioning pin 93 is covered up in FIG. 9 and is visible only in FIGS.11 and 13, which show the housing from below. This manner of fasteningmakes it possible, in any case, to realize a stable alignment of thelocator to the actuator.

After metal screen 60, equipped with printed circuit board 56, has beenfolded and inserted into housing 90, the housing is sealed by a cover100 depicted in FIG. 10. The cover 100 is provided with guide brackets101 to 104, which correspond to the inner sides of the housing.

The metal screen 60 is arranged between printed circuit board 56 andhousing 90 for reasons of electromagnetic compatibility, i.e., toprevent electromagnetic interference with the evaluation circuit and toprevent the emission of electromagnetic waves. As an alternative to theexemplary embodiment shown, an electromagnetic screen is obtained bymetallizing a plastic housing, or by using metal fiber-reinforcedplastic. To prevent impairment of the functioning of the GMR sensor,however, the material used may not have any ferromagnetic properties inthe area of the GMR sensor.

Printed circuit board 56, to improve protection and to make it suitablefor use in areas subject to explosion hazards, may be encapsulated withan insulating filler in housing 90.

The four-core cable 55, for connecting the locator with the controlunit, can also be single or double-shielded, depending on theapplication. The cable shield can easily be electrically connected withprinted circuit board 56 and/or the metallic screen 60.

FIGS. 11 to 14 illustrate the spatial arrangement of a magnet relativeto housing 90.

To detect angles of rotation, a magnet 94 is approximately centeredunder a substantially semi-circular opening 95 in housing 90. Thisopening 95 is a centering aid for adjusting the relative position ofmagnet 94 in relation to GMR sensor 50, which is located in housing 90.For this purpose, a positive locking positioning tool, which receivesmagnet 94, is inserted into opening 95. After the magnet has been firmlyfixed to a moving part (not depicted in FIGS. 11 and 12), magnet 94 iscentered and the positioning tool is removed. The axis of rotation ofmagnet 94 extends perpendicularly to the drawing plane in FIG. 11. Therotatability of magnet 94 is indicated by arrow 96. In FIG. 12, the axisof rotation extends through the center of magnet 94 in a horizontaldirection.

FIGS. 13 and 14 illustrate the arrangement of a magnet 97 for detectinglinear movements. This is indicated by a displacement arrow 98. In thiscase, magnet 97 together with GMR sensor 50, which is arranged inhousing 90, is located in a plane that is substantially perpendicular tothe above-described axis of rotation and parallel to the drawing planeof FIG. 13. A positioning tool, with a positive fit relative to opening95 and magnet 97, is again used for exact positioning of the magnet.

The selected arrangement of GMR sensor 50 in its housing 90 makes itpossible to use the same GMR sensor for detecting both angles ofrotation and linear motions without requiring any structural changes inits housing. Magnets 94 and 97 are held in a plastic component (notdepicted in the figures) and are encapsulated to protect them fromenvironmental influences. The guidance of magnets 94 and 97 in a movingpart (not depicted in the drawings) is structurally adapted to thecorresponding installation conditions, so that the rotary or linearmotion of an actuator is converted into a corresponding rotary or linearmotion of the magnets 94 or 97.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures disclosed. It is sought, therefore, to cover all such changesand modifications as fall within the spirit and scope of the invention,as defined by the appended claims, and equivalents thereof.

What is claimed is:
 1. A positioner, for a valve that is actuated by adrive, comprising: a locator that detects the actual position of anactuator, and a control unit that compares the actual position with apredefined desired position and generates an actuating signal, whereinsaid locator comprises a permanent magnet and a sensor, and wherein saidmagnet and sensor are rotatable or displaceable relative to one anotherin conjunction with a movement of said actuator, and further whereinsaid sensor is arranged in an area of a housing such that said sensor ispositioned to detect a relative rotation between said sensor and saidmagnet when said magnet rotates about an axis of rotation, and ispositioned to detect a relative shift between said sensor and saidmagnet when said magnet is displaceable, wherein said shift occurs in aplane that extends substantially perpendicularly to said axis ofrotation.
 2. The positioner as claimed in claim 1, further comprising atemperature compensation circuit arranged in said housing of saidsensor.
 3. The positioner as claimed in claim 2, wherein a bridgeresistance of said sensor provides a measurable resistance for saidtemperature compensation circuit.
 4. The positioner as claimed in claim2, further comprising a printed circuit board, wherein said sensor isarranged on a bottom side, and said temperature compensation circuit isarranged on a topside, of said printed circuit board.
 5. The positioneras claimed in claim 1, further comprising a centering aid arranged onsaid housing of said sensor, wherein said centering aid is capable ofadjusting the relative position of said magnet in relation to saidsensor during installation.
 6. The positioner as claimed in claim 1,wherein said control unit is arranged in a second housing that isseparate from said housing of said sensor.
 7. The positioner as claimedin claim 1, wherein said sensor is a GMR sensor.
 8. The positioner asclaimed in claim 1, wherein said sensor is an anisotropicmagnetoresistive sensor.